Keeping frisky bison in check

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CATALINA ISLAND The shaggy-headed bison looked downright laconic as they lounged near a watering hole – then a soft “crack” from a rifle, and the deed was done.

A contraceptive dart struck the rump of a bison cow, making her trot a few yards but otherwise causing only a small flurry among her companions.

Introduced in 1924 for the Zane Grey-based movie, “The Thundering Herd,” the Catalina bison have had plenty of time to grow accustomed to the armed intrusion of wildlife biologists. And the latest numbers show that birth-control by dart is turning into a major success story.

The birth rate dropped from 67 percent four years ago to 1.5 percent last year, and is holding the bison at a sustainable population of roughly 150.

While the method has been used for years by zoos, as well as on wild horses and African elephants, it's the first time contraceptive darting has proven effective on a wild bison herd.

“Applying this into a large, wild, free-ranging population is fairly new,” said wildlife biologist Calvin Duncan as he prepared his darts on a recent afternoon. “We're breaking ground a little bit here.”

A living laboratory

They hope their work could help other managers of bison herds across the country who might be considering using contraception. Biologists at Camp Pendleton are considering a similar approach for a herd of about 120 that lives on the Marine base.

Catalina, a kind of living laboratory, is home to plenty of quirky wildlife, from tiny island foxes to bald eagles.

Some became ecological basket cases, requiring intervention from scientists at the nonprofit Catalina Island Conservancy and the Institute for Wildlife Studies to help them survive.

The scientists helped the foxes fend off canine distemper with vaccinations, and also helped the eagles overcome eggshell thinning, a legacy of DDT once dumped off the Palos Verdes peninsula.

The original bison numbered around 14 and were left on the cutting room floor for “The Thundering Herd.” They once shared the island with feral goats and pigs, not to mention domestic cattle.

But those animals were removed or eliminated. Meanwhile, the bison population – treasured by the islanders and also valued as a major tourist attraction – exploded, threatening to overwhelm the island's vegetation.

The first answer involved packing up a portion of the bison population and shipping the animals off to Native American tribes or to private keepers.

But expense, as well as the wildly seesawing bison numbers, made that solution unworkable.

“The limitations were cost, potential stress to animals subjected to a larger population expansion between shipments, and change to social structure of the animals,” Duncan said.

One of the trickiest questions involved bison genes. Tests showed some of the Catalina bison at some point interbred with cattle, and some of those genes remained in the population.

In the early 2000s, part of the bison herd was shipped to the Standing Rock Sioux Reservation in South Dakota, viewed as a way to return the animals to their ancestral home.

But the evidence of interbreeding raised questions about whether the Catalina bison might spread cattle genes among their South Dakota counterparts.

So, conservancy biologists decided on contraception.

They injected the bison with a contraceptive called porcine zona pellucida, or PZP for short; its main feature is a pig-egg protein.

PZP doesn't sterilize the animals and doesn't manipulate their hormones. Instead, it provokes an immune response that blocks fertilization of bison eggs.

Reversing the effect is easy: Just stop dosing the bison and wait for the immune response to die down, and the bison can breed normally. The bison cows need two doses the first year, then a single shot each year in the weeks before breeding season begins.

The biologists try to allow the bison herd to behave as naturally as possible and using PZP prevents the disruption of normal breeding behavior that could result from hormone-based contraception.

“The social structure of the herd remains,” Duncan said. “The females still ovulate, the males still jockey and battle for the ability to breed.”

Only the cows are targeted. On this day, Duncan was on the hunt for four cows that he and wildlife biologist Julie King had previously identified.

Most of the island's bison are tagged, but Duncan – and especially King – are adept at recognizing individual bison by the shape of their horns or their facial characteristics.

Duncan carried his air rifle, with a long capsule of compressed carbon dioxide attached to control the force of the dart.

Duncan and King spoke by handheld radio as he slowly approached the herd of about 50 bison, which were standing in groups in a small depression amid hills.

King lagged behind, searching the herd through binoculars and directing Duncan toward the target females.

Accustomed to humans, the bison simply stared at Duncan or shuffled away as he drew nearer.

Once he got within about 35 yards, Duncan crouched, aimed his weapon and fired.

A small explosive charge inside the dart drives a plunger home once it hits the bison, then the dart drops off.

Duncan planted a flag to mark the spot he'd fired from, which helps the scientists recover the dart.

In quick succession, three flags were planted, but the bison were becoming increasingly restless.

King zeroed in on the fourth cow and directed Duncan in her direction. But the cow kept her distance, eventually disappearing with her companions over a ridge.

But she didn't escape them for long, and the scientists managed to dart the final bison of the season.

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